The present invention relates to a process for operating the vapor phase epoxidation of ethylene in the presence of a supported Highly Selective silver based catalyst. More particularly, it relates to an improved process for ethylene epoxidation wherein, in the course of operation, consequences of catalyst aging are offset.
In the catalytic epoxidation of ethylene, modern silver-based supported catalysts are highly selective towards ethylene oxide production. Under certain operation conditions their selectivity towards ethylene oxide, expressed as a percentage of the ethylene converted, can reach values above the {fraction (6/7)} or 85.7 mol % limit which formerlyxe2x80x94based on the reaction formula 7 C2H4+6O2xe2x86x926C2H4O+2CO2+2H2O, see Kirk-Othmer""s Encyclopedia of Chemical Technology, 3rd ed. vol 9 (1980) p. 445xe2x80x94was considered to be the theoretically maximal selectivity of this reaction. Such Highly Selective catalysts, comprising as their active components silver, rhenium, at least one further metal and optionally a rhenium co-promoter, are disclosed in EP-B 0 266 015 and in several subsequent patent publications.
Like all catalysts, the Highly Selective silver based ethylene epoxidation catalysts are subject to aging-related performance decline during normal operation and they need to be exchanged periodically. The aging manifests itself by a reduction in both selectivity and activity performance of the catalyst. Selectivity and activity are the primary (although not the only) determinants of plant profitability. There exists, therefore, a considerable economic incentive for delaying the need for exchanging the catalyst by preserving these values as long as possible. Several patent publications are known which are directed at stabilizing the catalyst by introducing modifications in the catalyst composition or in the support material, but thus far the reaction conditions and, in particular the feed composition, escaped attention in this respect.
It is known in general that when the concentration of ethylene and/or of oxygen in the reactor feed gas is increased, both activity and selectivity of the ethylene epoxidation reaction can improve.
In EP-A 567 273 there is disclosed a process for the production of ethylene oxide in the presence of a silver metal catalyst and halide gas phase inhibitor in a reaction zone, characterized by the introduction into the reaction zone of a feed gas mixture comprising 30 to 90 mol % ethylene, 2 to 10 mol % oxygen and 1 to 50 ppm organic halide gas phase inhibitor, the temperature in the reaction zone being maintained between 180 and 350xc2x0 C. The Examples of the reference show a small improvement in selectivity and a greater improvement in activity occurring during 15 to 20 operation hours, as the concentration of ethylene is raised from 30 mol % through 45 and 60 mol % up to 75 mol %xe2x80x94the concentration of oxygen being kept constant at 8 mol % in all cases and the composition of the catalyst not being specified.
However, it is also known that in actual practice in order to remain outside the flammability limit of the gas mixture the concentration of oxygen has to be lowered as the concentration of ethylene is raised. The actual safe operating ranges depend, along with the gas composition (reactants and balance gases), also on individual plant conditions such as temperature and pressure. Therefore in each individual plant a so-called flammability equation is used to determine the concentration of oxygen which may be used with any given concentration of ethylene. This flammability equation can be expressed graphically in a so-called flammability curve.
The fact that in the examples of EP-A 0 567 273 the oxygen concentration was kept constant while the ethylene concentration was raised makes these examples unrealistic as regards actual plant conditions. It implies that either the oxygen concentration used with the lowest ethylene concentration level was sub-maximal (in which case the performance at this lowest ethylene concentration level was actually sub-optimal), or the oxygen concentration used with the higher ethylene levels was above the flammability limit (in which case the improved experimental results cannot be carried over to commercial plant operation). Either way, this may throw some doubt on the relevance of the showing of higher performance at the higher ethylene concentrations, since the different ethylene concentrations exemplified were not compared at their respective maximum allowable oxygen levels. Moreover it is to be noted that fresh catalysts of an unspecified composition were used in these examples, during only 15 or 20 operating hours.
It has now been found that aged Highly Selective ethylene oxidation catalysts react differently to the composition of the reactant gas mixture than do fresh Highly Selective ethylene oxidation catalysts. More specifically, with the fresh Highly Selective catalysts the selectivity of the reaction towards ethylene oxide is not influenced substantially when a higher concentration of ethylene is combined with a lower (i.e. safe) concentration of oxygen, while with the aged Highly Selective catalysts the selectivity under these conditions is substantially improved. Differences of activity performance under conditions of raised ethylene concentration and lowered oxygen concentration between fresh and aged Highly Selective catalysts are in the same direction but less pronounced. By contrast to the highly selective catalysts it has been found that aged and fresh traditional ethylene oxidation catalysts, i.e. catalysts whose selectivity does not reach the level of {fraction (6/7)} or 85.7%, do not exhibit this clear difference in their reaction to the composition of the feed gas mixture.
The present invention therefore provides a process for the vapor phase oxidation of ethylene to ethylene oxide in the presence of a supported highly selective silver-based catalyst, at a work rate w in the range of from 32 to 320 kg ethylene oxide produced per m3 of catalyst per hour, the reaction mixture containing ethylene, oxygen, optional carbon dioxide, gas phase moderator and balance inert gases, the reaction temperature being from 180 to 325xc2x0 C., the reactor inlet pressure from 1000 to 3500 kPa and the GHSV from 1500 to 10000, the process comprising:
operating at an initial operation phase wherein fresh catalyst is used, the reaction gas mixture containing an ethylene concentration which represents an economically optimized balance between catalyst performance (expressed, at the given work rate w, by the selectivity Sw in mol % and by the operating temperature Tw in xc2x0 C.) on the one hand and ethylene vent losses on the other, and an oxygen concentration which complies with safety-related flammability restrictions; and
operating at a further operation phase when the catalyst has reached an advanced aged defined by a cumulative ethylene oxide production exceeding 1.5 kT EO per m3 of catalyst, wherein in said further operation phase the composition of the reaction mixture is changed to contain from 1.1 to 4 times the concentration of ethylene used in the initial operation phase and the corresponding optimized and safe concentration of oxygen.
By raising the ethylene content of the reaction gas mixture, yet simultaneously reducing the oxygen content to remain below the flammability limit, both selectivity and activity of the aged High Selectivity catalyst are improved significantly.